The recent proliferation of Wi-Fi access points in wireless local area networks (WLANs) has made it possible for navigation systems to use these access points for position determination, especially in areas where there are a large concentration of active Wi-Fi access points (e.g., urban cores, shopping centers, office buildings, and so on). In WLAN positioning systems, the locations of Wi-Fi access points (APs) are used as reference points from which well-known trilateration techniques can determine the location of a mobile wireless device or station (e.g., a Wi-Fi-enabled cell phone, laptop, or tablet computer). For example, the wireless station (STA) can use the received signal strength indicators (RSSI) associated with a number of visible APs as indications of the distances between the mobile device and each of the detected APs, where a stronger RSSI means that the mobile device is closer to the AP and a weaker RSSI means that the mobile device is further from the AP. The STA can also use the round trip time (RTT) of signals transmitted to and from the APs to estimate the distances between the STA and the APs. Once the distances between the STA and at least three APs are calculated, the location of the STA relative to the APs can be determined using trilateration techniques.
WLAN positioning systems are typically controlled by a central server that can instruct APs associated with the WLAN to perform ranging operations with a STA and then report the resulting ranging measurements back to the server. For example, an AP can initiate a ranging operation with the STA by sending a NULL frame to the STA, which in response thereto sends an acknowledgement (ACK) frame back to the AP. The AP can use the difference between the time of departure (TOD) of the NULL frame and the time of arrival (TOA) of the ACK frame to calculate a round trip time (RTT) value of the exchanged NULL and ACK frames. Then, the RTT value can be correlated to a distance.
However, if the AP probes the STA when the STA is in a power save mode, the STA may not receive the NULL frame sent by the AP, in which case the STA will not respond with an ACK frame sent back to the AP. Thus, ranging operations may be difficult to initiate when the STA is in the power save mode. However, it is even more difficult for other APs not associated with the STA to initiate such ranging operations with the STA when the STA is in the power save mode. For example, according to the IEEE 802.11 family of standards, a STA can be “associated” with only one AP at any given time. Thus, the AP with which the STA currently has an established wireless communication channel or link is commonly referred to as the associated AP, and all other APs (which do not have currently have an established wireless communication channel or link with the STA) are commonly referred to as “non-associated” APs. Although these “non-associated” APs can spoof the MAC address of the associated AP and then send spoofed NULL frames to the STA to initiate ranging operations, these non-associated APs typically do not know whether the STA is in the power save mode. Thus, unless the STA coincidentally wakes up from power save mode precisely when these non-associated APs send spoofed NULL frames to the STA, the STA will not receive the spoofed NULL frames and, therefore, will not respond with ACK frames sent back to the spoofing AP. Accordingly, such spoofing techniques frequently fail to successfully initiate ranging operations with a STA that is in power save mode.
Thus, there is a need for WLAN positioning system to allow any of its APs to initiate ranging operations with the STA, regardless of whether the STA is in power save mode and regardless of whether the ranging AP is currently associated with the STA.